broken_scope.js

#! /usr/bin/env node

var foo = function() {
    console.log("foo's x: %s BEFORE IT ENTERS SCOPE", x);
    x = "xb";
    y = "ya";
    return 0
}

var main = function() {
    x = "xa";
    console.log("main's x1: %s", x);
    foo();
    console.log("main's x2: %s", x);
    console.log("main's y: %s, WHICH IT HAS NEVER SEEN BEFORE", y);
}

main()

$ ./broken_scope.js
main's x1: xa
foo's x: xa BEFORE IT ENTERS SCOPE
main's x2: xb
main's y: ya, WHICH IT HAS NEVER SEEN BEFORE

This is a very easy to make mistake, especially in loops: https://gist.github.com/xandkar/9a422ea5cf01408a9d67fa4800e53b25

#! /usr/bin/env node

var foo = function() {
    console.log("foo's x: %s BEFORE IT ENTERS SCOPE", x);
    var x = "xb";
    var y = "ya";
    return 0
}

var main = function() {
    x = "xa";
    console.log("main's x1: %s", x);
    foo();
    console.log("main's x2: %s", x);
    console.log("main's y: %s, WHICH IT HAS NEVER SEEN BEFORE", y);
}

main()

this is possible because you are in non strict mode.
And because of this, xand y are attached to the global scope.
if you test console.log(global.x) and console.log(global.y) you may find the same result.
non-strict mode is a nightmare because of scope leaks like this.
And this is the same with this.
Meaning: with node, always activate strict mode. With browsers, if you don't depend on IE 8, activate it as well.
http://caniuse.com/#search=strict%20mode

@rehia @AeroNotix: Yep. See the gist about loops, which includes output of versions with and without "use strict": https://gist.github.com/xandkar/9a422ea5cf01408a9d67fa4800e53b25

:)